The present embodiment relates generally to a treating fluid, particularly a drilling fluid, for introduction into a subterranean zone penetrated by a wellbore.
Conventionally, a wellbore is drilled into a subterranean zone using a drilling fluid that is circulated through the wellbore. During the drilling of a wellbore, the drilling fluid is continuously circulated down the drill pipe, through the drill bit, and back to the surface through the annulus between the drill pipe and the walls of the wellbore. After a wellbore has been drilled to total depth, the circulation of the drilling fluid is stopped (called a “shut-down period”), while the well is logged and pipe is run in the wellbore.
One function of a drilling fluid is to seal off the walls of the wellbore so that the fluid is not lost into highly permeable subterranean zones penetrated by the wellbore. During the shut down period, this is accomplished by the deposit of a filter cake of solids from the drilling fluid, and additional dehydrated drilling fluid and gelled drilling fluid, on the walls of the wellbore.
After the pipe is run in the wellbore, the drilling fluid in the wellbore is cleaned up by re-initiating circulation of drilling fluid. The drilling fluid is circulated downwardly through the interior of the pipe and upwardly through the annulus between the exterior of the pipe and the walls of the wellbore, while removing drilling solids, gas, filter cake, dehydrated drilling fluid, gelled drilling fluid, cuttings, and any other debris needing to be removed from the wellbore. Thus, a drilling fluid must be capable of transporting a sufficient amount of cuttings and other debris through the annulus and up to the surface in order to provide a clean wellbore for subsequent cementing operations.
After the wellbore is cleaned by the drilling fluid, primary cementing operations are performed therein. Namely, the pipe is cemented in the wellbore by placing a cement slurry in the annulus between the pipe and the walls of the wellbore. The cement slurry sets into a hard impermeable mass, and is intended to bond the pipe to the walls of the wellbore whereby the annulus is sealed and fluid communication between subterranean zones or to the surface by way of the annulus is prevented.
As a result of the polymeric viscosifiers and additives typically used in drilling fluids, the filter cake formed is generally very stable and can be difficult to remove. However, removal of the gelled and dehydrated drilling fluid and filter cake from the walls of the wellbore and displacement of the drilling fluid from the wellbore must take place prior to primary cementing operations in order to achieve a satisfactory bond between the pipe, primary cement and the walls of the wellbore.
Heretofore, attempts have been made to remove the drilling fluid and filter cake from the wellbore by attaching mechanical scrapers to the pipe so that as the pipe is run into the well bore, it physically contacts and breaks up some of the drilling fluid and filter cake. In addition, flushes are normally run through the annulus between the pipe and the walls of the wellbore prior to cementing in order to remove drilling fluid and filter cake therein. It is also known to use spacer fluids in oil and gas wells to displace drilling fluids and remove filter cake deposits from the walls of the wellbore. Other methods for removing drilling fluid and filter cake from the wellbore include pumping fluids through the annulus at high rates so that they are in turbulence as they contact the filter cake, and including surfactants in flush fluids to lower surface tension and enhance the penetration of the flush fluids into the filter cake. If appreciable drilling fluid and filter cake remain on the walls of the wellbore, primary cementing operations are less than satisfactory, as the cement will not properly bond to the walls of the wellbore and fluid leakage through the annulus and other major problems can result.
Still other methods for achieving satisfactory primary cementing operations when deposits of filter cake are an issue include laying down a settable filter cake on the walls of the wellbore and activating the filter cake to harden and set up. Thus, any remaining filter cake is less likely to interfere with primary cementing operations.